The clay-sized mineral assemblage in twelve deep-sea sediment cores from the northern part of the Magellan Trough in the Central Pacific shows systematic and homogenous changes throughout the Cenozoic. Smectite in the present core samples is considered to have been formed in sit-subsea in most cases. The most probable mode of formation of the authigenic smective was the diagenetic change of a precurosor such as volcanic glass. In this case, Si derived from biogenic silica must have contributed to the mechanism of the formation. The coexistence of clinoptilolite with smectite supports this hypothesis. The formation of authigenic smectite and dinoptilolite continued through the Paleogene and into the Miocene, even though clastic clay minerals such as illite, chlorite, and kaolinite tend to increase in younger sediments. The inflow of these clastic clay-sized minerals by air to the study remarably increased during the Pliocene owing to global climatic change. The WNW movement of the Pacific plate must have playd a role in receiving more clastic cla-sized minerals. The presence of hiatuses in the present cores suggests that AABW was unable to deposit clay-sized minerals at times in the study area.